In gas turbines it is desirable to reduce gas leakage around the turbine blades in order to improve the efficiency of the turbine. This can be achieved by surrounding each array of turbine blades with a ring of abradable material. As the turbine rotates the tips of the turbine blades cut a path through the abradable material, so ensuring that only a very small gap is left between the turbine blade tips and the surface of the abradable material. Since this gap is very small, leakage is restricted.
Unfortunately, in the extreme environment found within the turbine, the abradable material tends to erode over time. As a result, it is desirable to replace the abradable material at intervals. In order to simplify replacement, the abradable material is supported by shroud liners. These shroud liners are in turn attached to the structural casing of the turbine. Furthermore the shroud liners are circumferentially segmented to make assembly simpler, allow individual areas of the lining to be replaced, and to accommodate better any distortions caused by the extreme temperatures within the turbine.
It is necessary to mount the shroud segments to the structural casing so that they are held accurately relative to the blade tips. This is important since any movement is likely to increase the clearance at the blade tips, so increasing leakage. The mounting is either directly from the casing, from the stationary nozzle guide vane assemblies which precede and follow the turbine rotor and are themselves fixed to the casing, or from a combination of both.
U.S. Pat. No. 6,062,813 describes a known arrangement for mounting the shroud segments to the casing. In the disclosed arrangement, a shroud liner is made up of an annular array of circumferentially abutting shroud liner segments each of which has a first positive radial location means and a second location means to locate each segment within the casing. The location means are in the form of hooks. The first hook is arranged to enable axial insertion of the shroud segment between a bladed rotor and the casing, and the second is arranged to retain the segment in position allowing a limited amount of radial translation of the shroud segment during axial insertion of the segment. When the shroud segment is located in its desired position, the second hook provides a positive radial location to prevent radial translation of the shroud segment.
Whilst well suited to the end purpose, it has been found difficult to monitor the clearance between the shroud and the tip of the rotor blade. Consequently, there can be uncertainty as to the clearance achieved after assembly.